This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-090953, filed Mar. 27, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a magnetic material and, more particularly, to a magnetic material capable of realizing magnetic refrigeration using a relatively low magnetic field in near room temperature region.
2. Description of the Related Art
Presently, a gas compression/expansion cycle is primarily used in refrigeration systems, e.g., refrigerators, freezers, and air-conditioners, closely related to daily human life. However, this gas compression/expansion cycle is environmentally problematic, in terms of the exhaustion of specific freon gases. In addition, substitute freon gases presumably have a bad effect on the environment. From this background, clean and efficient refrigeration technologies causing no environmental problems due to wastage of operating gases are being demanded to put into practical use.
Recently, magnetic refrigeration is being increasingly expected as one such environment-friendly, highly efficient refrigeration technology. Much research and development of the related technologies for use in near room temperature region has been undertaken. Magnetic refrigeration generates low temperatures as follows by applying the magnetocaloric effect (a phenomenon in which when an external magnetic field is changed with respect to a magnetic material while the magnetic material is thermally insulated, the temperature of this magnetic material changes).
The entropy of a magnetic material is changed depending on whether a magnetic field is applied or not, owing to the difference between the degrees of freedom of the electron spin system. With this entropy change, the entropy transfers between the electron spin system and the lattice system. Magnetic refrigeration uses a magnetic material having a large electron spin and produces a large entropy change between the magnetic field applied state and the magnetic field removed state. Using this large entropy change, magnetic refrigeration generates low temperatures.
Note that in a temperature region of 1 K or more, a xe2x80x9cmagnetic materialxe2x80x9d usually means a substance which shows magnetism due to an electron spin. On the other hand, in a temperature region of a few mK or less, magnetism due to a nuclear spin becomes relatively large. For example, an ultra low temperature of 27 xcexcK was reportedly generated by using PrNi5. In the following description, a xe2x80x9cmagnetic materialxe2x80x9d means a substance showing magnetism due to an electron spin, and a target temperature region is 1 K or more, unless otherwise specified.
In the early 1900s, magnetic refrigeration systems using paramagnetic salts such as Gd2(SO4)3.8H2O and paramagnetic compounds represented by Gd3Ga5O12 (gadolinium gallium garnet xe2x80x9cGGGxe2x80x9d) were developed. However, these magnetic refrigeration systems using paramagnetic materials were in most cases applied to an low temperature region of 20 K or less for the following reason. That is, as the lattice vibration increases with an increase in temperature, the higher magnetic field is required to reduce the lattice vibration by entropy transferring between electron spin system and lattice system. Therefore, assuming a magnetic field of about 10 tesla which can be obtained using an ordinary type superconducting magnet, temperatures at which magnetic refrigeration can be achieved using paramagnetic materials are limited to the low temperature region of 20 K or less.
On the other hand, to realize magnetic refrigeration at higher temperatures, the research of magnetic refrigeration using the magnetic phase transition of a ferromagnetic material between a paramagnetic state and a ferromagnetic state was extensively made after the 1970s. As a consequence, a large number of magnetic materials containing rare earth elements having a large electron magnetic spin per unit volume have been proposed. Examples are lanthanoide rare earth elements such as Pr, Nd, Dy, Er, Tm, and Gd, rare earth alloy materials containing two or more rare earth elements such as Gdxe2x80x94Y and Gdxe2x80x94Dy, and rare earth intermetallic compounds such as RAI2 (R represents a rare earth element, and this similarly applies to the following description), RNi2, and GdPd.
When any of these ferromagnetic substances are used, an external magnetic field is applied at a temperature close to the ferromagnetic phase transition temperature (Curie temperature; Tc), thereby causing the magnetic phase transition of electron spin system from a paramagnetic state to a ferromagnetic ordered state. Magnetic refrigeration is realized by using the resulting entropy change. Therefore, the applicable temperature range is limited to the vicinity of the ferromagnetic phase transition temperature (Tc) of each magnetic material. However, the magnitude of the external magnetic field need only be the one enough to assisting a magnetic phase transition. A magnetic field of this magnitude can be well generated in a temperature region much higher than 20 K.
In 1974, Brown (U.S.A.) achieved magnetic refrigeration at room temperature for the first time, by using a ferromagnetic substance Gd plate having a ferromagnetic phase transition temperature (Tc) of about 294 K. Unfortunately, although the refrigeration cycle was continuously operated in the experiment, there were some problems such as heat transfer in a refrigeration cycle, because an integral Gd plate was used in the range of the hot end to the cold end.
Magnetic refrigeration in the range of intermediate temperatures much higher than 20 K to room temperature has a substantial problem. That is, lattice vibration becomes large as the temperature rises, so, in a temperature region of 100 to 150 K or more the lattice system entropy becomes large compared with the magnetic entropy of the electron spin system. Accordingly, even when the entropy is exchanged between the electron spin system and the lattice system by changing the magnitude of the external magnetic field, the magnetocaloric effect, i.e., a temperature reduction (xcex94Tad) of the magnetic substance is small.
In 1982, Barclay (U.S.A.) attempted to use the lattice entropy positively that had been regarded as an interference to magnetic refrigeration in the range of intermediate temperatures to room temperature (or a temperature range in which the lattice entropy is large relative to the magnetic entropy), and proposed a method of refrigeration (U.S. Pat. No. 4,332,135) in which a magnetic material is used, in addition to magnetic refrigeration by the magnetocaloric effect, as the regenerator for storing coldness generated by the refrigeration. This magnetic refrigeration method is called AMR (xe2x80x9cActive Magnetic Refrigerationxe2x80x9d).
In 1997, Zimm, Gschneidner, and Pecharsky of the U.S.A. built a prototype AMR machine using a packed column filled with fine spherical Gd, and succeeded in a continuous steady-state operation of the magnetic refrigeration cycle at room temperature (xe2x80x9cAdvances in Cryogenic Engineeringxe2x80x9d, Vol. 43, 1998). According to this reference, refrigeration at about 30xc2x0 C. was accomplished by changing the magnitude of the external magnetic field from 0 to 5 tesla by using a superconducting magnet at room temperature. When the refrigerating temperature difference (T) between the hot end and the cold end was 13xc2x0 C., a very high refrigeration efficiency (COP=15; excluding the power input to the magnetic field generating means) was reportedly obtained. Note that the refrigeration efficiency (COP) of a gas compression/expansion cycle (e.g., a household refrigerator) using conventional freon is about 1 to 3.
In addition to the above-mentioned technical demonstration of the AMR-cycle magnetic refrigeration system using Gd, Pecharsky and Gschneidner of the U.S.A. developed a Gd5(Ge,Si)4-based material as a magnetic material from which a very large entropy change can be obtained at room temperature (U.S. Pat. No. 5,743,095). For example, Gd5(Ge0.5Si0.5)4 shows an entropy change (xcex94S) of about 20 J/(kgxc2x7K) when the magnitude of the external magnetic field is changed from 0 to 5 tesla at about 277 K, and shows an entropy change (xcex94S) of about 15 J/(kgxc2x7K) when the magnitude of the external magnetic field is changed from 0 to 2 tesla. That is, a large entropy change twice or more that of Gd is observed near room temperature.
Unfortunately, in the experiments conducted by Zimm, Gschneidner, and Pecharsky described above, a superconducting magnet was used to apply a large external magnetic field of about 2 to 5 tesla to Gd as a magnetic material for magnetic refrigeration. Since under the present conditions a cryogenic environment at about 10 K is necessary to operate a superconducting magnet, the system increases in size. In addition, when a superconducting magnet is to be used, it is necessary to use a freezing medium such as liquid helium or a refrigerator for cryogenic generation. It is impractical to apply a system like this to ordinary purposes such as refrigeration and air-conditioning.
A heavy duty electromagnet is another means for generating a large magnetic field, other than a superconducting magnet. When this electromagnet is to be used, however, a large input current and water cooling against Joule heating are necessary. This makes the system larger and also increases the operation cost. Accordingly, similar to the case of a superconducting magnet, it is impractical to apply a system using an electromagnet for usual purposes.
A permanent magnet is a small convenient magnetic field generating means. However, it is difficult to generate a large magnetic field of about 2 to 5 tesla using such a magnet. According to the reported results of experiments using an NdFeB-based permanent magnet and Gd as a magnetic material for magnetic refrigeration, the cooling temperature at room temperature is very low, 1.6xc2x0 C., because the magnitude of the magnetic field is small. This greatly differs from the refrigerating ability of the conventional gas compression/expansion cycle.
The present invention has been made in consideration of the problems of the magnetic refrigeration technologies in near room temperature region. It is an object of the present invention to provide a magnetic material for magnetic refrigeration, by which magnetic refrigeration can be realized using a relatively low magnetic field.
The magnetic material of the present invention is characterized by exhibiting, in a certain temperature region (only in a partial temperature region), an inflection point at which a second order differential coefficient of a magnetization curve changes from positive to negative with respect to a magnetic field, within the range of the strength of the magnetic field obtained using a permanent magnet.
Preferably, the magnetic material of the present invention is characterized by exhibiting, only in part of the temperature region from 200 K to 350 K, the above-mentioned inflection point on a magnetization curve within the range of the strength of a magnetic field of 1 tesla or less.
In the present invention, an external magnetic field is applied using a permanent magnet unit, near a temperature indicating the inflection point, to a magnetic material having a magnetization curve meeting the above condition. Magnetic refrigeration can be realized by transferring entropy between the electron spin system and the lattice system by changing the magnitude of the external magnetic field.
The inventors of the present application found that within the range of near room temperature from 200 K to 350 K (i.e., temperatures closely related to everyday life, e.g., from the temperature of dry ice to that of hot water), and within the range of the strength of a relatively low magnetic field of 1 tesla or less, it is effective to urge a ferromagnetic interaction and an antiferromagnetic interaction to compete with each other, as a means for obtaining the inflection point as described above on a magnetization curve.
The reason why the above-mentioned inflection point appears on a magnetization curve is presumably as follows. By urging a ferromagnetic interaction and an antiferromagnetic interaction to compete with each other, several electronic states having close energy levels are formed. The relation of the energy levels of each electronic states changes in accordance with the amplitude of the external magnetic field. Hence, upon application of a magnetic field, the magnetic spin configuration partly or entirely changes inside the material system. As a consequence, the inflection point appears on a magnetization curve.
The important point is that in near room temperature region from 200 K to 350 K, several electronic state is in very close energy level. A large entropy change cannot be obtained, i.e., efficient magnetic refrigeration cannot be realized by the application of a relatively low magnetic field, unless this special condition is formed.
In addition, when magnetic refrigeration is actually performed using a magnetic material, not only the magnitude of an entropy change xcex94S(T, xcex94H) corresponding to a magnetic field change (xcex94H), but also a temperature range with which the peak of the entropy change appears is an important factor. That is, even when a large entropy change is obtained, if this entropy change is obtained only within a very narrow temperature range (e.g., about 1 to 2 K), the magnetic refrigeration cycle cannot be stably operated. More specifically, a stable refrigeration cycle cannot be achieved, or even if it can be achieved, is impractical for use in a refrigerator.
In the case of the AMR, for example, a magnetic material works as not only the magnetic refrigerant but also the regenerator, so a temperature gradient is generated inside a magnetic refrigeration chamber when the refrigeration cycle is operated in a steady state. That is, even when the temperature of the magnetic material is almost uniform in the magnetic refrigeration chamber at the beginning of the operation, a temperature gradient is gradually formed in the magnetic refrigeration chamber as the refrigeration cycle is repeated. This makes the two end portions of the magnetic refrigeration chamber become hot and cold ends. Consequently, the magnetic material in the magnetic refrigeration chamber operates at temperature cycles of different ranges at different positions. In a steady-state operation, these temperature cycles are also in a steady state. If the magnetic material is the one with which the peak of an entropy change appears only within a very narrow temperature range (e.g., 1 to 2 K), a refrigeration cycle is achieved only within this narrow temperature range. Accordingly, it is difficult to perform stable operation in a large refrigeration temperature difference (e.g., 10 K to 20 K or a larger range) using such a material.
Note that it is also possible to arrange magnetic materials exhibiting the peak of an entropy change in different temperature regions, in accordance with this temperature gradient during a steady-state operation, from the hot end to the cold end of the magnetic refrigeration chamber. However, the steady state is gradually approached through different temperature cycles as the refrigeration cycle is repeated from the start of operation. Therefore, each magnetic material must be a substance by which an entropy change appears within a temperature range wider than the temperature amplitude during steady-state operation.
For the reasons described above, a magnetic material for magnetic refrigeration must have a large entropy change and a wide temperature range (width) over which a peak appears in an entropy change. Note that the temperature width of the peak of an entropy change means the bottom width of the peak, not the half-width. This is so because the effective temperature width of the peak has an effect in an actual temperature cycle. This effective temperature width of the peak is a peak width obtained by removing an error level from the bottom portion.
Letting xcex94S(T, xcex94H) be an entropy change (temperature dependence) at a temperature T with respect to a specific external magnetic field change xcex94H and xcex94Smax be the peak value of xcex94S, the effective temperature width of the peak is defined as follows: the range of the temperature T within which xcex94S(T, xcex94H) greater than 0.1*xcex94Smax, when a value which is {fraction (1/10)} of xcex94Smax is a bottom level reference, or, if 0.1*xcex94Smax greater than 1[J/(kg,K)] is met, the range of the temperature T within which xcex94S(T, xcex94H) greater than 1[J/(kg,K)], regarding 1[J/(kg,K)] as a bottom level reference.
To realize a magnetic refrigeration cycle by using a single magnetic material, the effective temperature width of the peak of the entropy change xcex94S(T, xcex94H) must be 3 K or more. This effective temperature width is preferably about 5 K or more, and more preferably, 8 K or more.
Additionally, the peak of the entropy change described above preferably has no temperature hysteresis. Even if the peak has a temperature hysteresis, this hysteresis is 8 K or less, preferably, 3 K or less, and more preferably, 1 K or less.
However, the research to-date reveal that an entropy change and the temperature width of its peak often have a tradeoff relationship. Therefore, it is important to obtain a good balance between them.
If the peak width of an entropy change is as narrow as about 1 to 2 K, giving a slight composition fluctuation is effective as a means for widening the peak width so that the peak width becomes a practical one. By giving this slight composition fluctuation, it is possible to slightly change the energy level balance of electronic state in microscopic portions, without largely changing the physical characteristics of a magnetic material, and to distribute a temperature at which the inflection point appears within a microscopic region. As a consequence, the peak width of an entropy change in a magnetic material can be increased.
When the metal texture is taken into consideration, the peak width of an entropy change can also be increased, without largely changing the physical characteristics of a magnetic material as described above, by precipitating a small amount of a second phase, having a crystal structure different from that of a main phase, with respect to this main phase. Practically no problem arises if the amount of this second phase is 30 vol % or less. This can increase the peak width of xcex94S.
Examples of practical methods are to use a preparation composition slightly different from a predetermined composition, slightly add additional elements, and rapidly cool a metal in a molten state during synthesis.
To obtain a large entropy change, the internal system of magnetic material must have large degree of freedom. To increase the degree of freedom of internal magnetic system of a magnetic material, it is preferable to use transition metal elements such as Fe, Ni, Co, Mn, and Cr, or rare earth elements such as Pr, Nd, Gd, Tb, Dy, Er, Ho, or Tm, as main constituent elements.
Furthermore, to obtain the inflection point as described above on a magnetization curve within the range of a magnetic field of 1 tesla or less in near room temperature region of about 200 K to about 350 K, it is effective to add a total of 50 atomic % or more of one or more of Fe, Ni, Co, Mn, and Cr. This is so because, if the ratio of a transition metal element such as Fe is low, it becomes difficult to make the above inflection point appear in a high-temperature region of 200 K or more with a magnetic field of 1 tesla or less.
When Gd, Sm, or Tb having a relatively strong magnetic interaction among other rare earth elements is used, the total amount of this element and a transition metal element such as Fe, Co, Ni, Mn, or Cr enumerated above is preferably 60 atomic % or more, in order to make the above inflection point appear at a temperature of 200 K or more.
A magnetic material meeting the above condition is, e.g., a magnetic material which comprises
a total of 50 to 96 atomic % of one element or two or more elements selected from the group consisting of Fe, Co, Ni, Mn, and Cr,
a total of 4 to 43 atomic % of one element or two or more elements selected from the group consisting of Si, C, Ge, Al, B, Ga, and In, and
a total of 4 to 20 atomic % of one element or two or more elements selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb.
In the second group, Si or Ge is preferred, and 4 to 25 atomic % of Si are particularly preferred.
Representative examples of this magnetic material are R(T,M)13, R(T,M)12, R2(T,M)17, and R3(T,M)29 (R is a rare earth element, T is a transition element, and M is the above element of group 3B or 4B). This magnetic material is particularly preferably (La,Pr,Ce,Nd)(Fe,T,Si)13 or (La,Pr,Ce,Nd)(Fe,T,Si,M)13.
Another magnetic material meeting the above condition is, e.g., a magnetic material which comprises
a total of 60 to 96 atomic % of one element or two or more elements selected from the group consisting of Fe, Co, Ni, Mn, and Cr, and
a total of 4 to 40 atomic % of one element or two or more elements selected from the group consisting of Sc, Ti, Y, Zr, Nb, Mo, Hf, Ta, and W. Note that two or more elements are particularly preferably selected from the second group.
In this second group, Ti, Zr, Nb, and Hf are particularly favorable, and their total amount is preferably 25 atomic % or more.
A representative example of this magnetic material is (Hf,Ta)Fe2, (Ti,Sc)Fe2, and (Nb,Mo)Fe2.
From a viewpoint of practical use, a magnetic material for magnetic refrigeration should better exhibit large change in magnetic entropy per weight or per volume to minimize the weight and volume of refrigeration systems. In addition, from another viewpoint of practical use, it should preferably exhibit large change in magnetic entropy per unit magnetic moment. The reason is as follows. In the gradient of magnetic field, a magnetic material is affected by external force (magnetic force) which is proportional to the magnetic moment of the material. The magnetic force is one of the disturbance factors in practical use when the relative position between the magnetic material and the permanent magnet is controlled.
A magnetic material meeting the above condition is, e.g., a magnetic material which comprises
a total of 50 to 80 atomic % of one or not less than two elements selected from the group consisting of Fe, Co, Ni, Mn, and Cr,
a total of 20 to 50 atomic % of one or not less than two elements selected from the group consisting of Sb, Bi, P, and As.
Representative examples of this magnetic material are (Mn,Cr)2(Sb,As,P), (Mn,Cr)(Sb,As,P,Bi), (Co,Mn,Fe,Ni)2(P,As), and (Fe,Co,Mn)3P. This magnetic material is particularly preferably (Mn,Cr)2Sb, (Mn,Cr)Sb, (Co,Mn)2P, and (Fe,T)2(P,As).
To control the electronic state subtly, it is effective to substitute a part (below 10%) of 3B transition element such as Fe, Co, Ni, Mn or Cr with 4B transition element such as Rh or Pd. In addition, a part (below 20%) of 5B transition element such as Sb, Bi, P and As is substituted for light element such as B or C, to control the electronic state subtly.
If the content of oxygen is large in the manufacture of the above magnetic material, this oxygen and a metal element combine to form a refractory oxide in a melting step (of melting and mixing materials). This oxide floats as a refractory impurity in the molten metal layer, and reduces the quality of the material manufactured in the melting step and resolidification step. To minimize the formation of this oxide, therefore, the oxygen content is preferably decreased to 1 atomic % or less.
In the magnetic material of the present invention, the inflection point as described above appears within the range of a relatively low magnetic field. Accordingly, a magnetic refrigeration system can be realized by using a small permanent magnet unit, without using any superconducting magnet or electromagnet having a large current capacitance, near a temperature at which the inflection point appears.